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Quick answer
Buy a harness that fits your Border Collie puppy's chest today, not the size it will be as an adult1. Measure every couple of weeks while it grows fast, and move up a size when its chest reaches the top of its current size.
For comparison, an adult Border Collie usually has a chest of about 55–75 cm (our estimate) and weighs 12–20 kg2. Those adult sizes are shown below so you know where your puppy is heading, not what to buy now.
Adult sizes: Julius-K9 IDC Powerharness: Mini or 0; Ruffwear Front Range Harness: XS to M; Bailey & Coco Trail & Glow Harness: M or L.
Enter your puppy's chest measurementSizing everything else too? Read sizing your puppy through the first year: collar, harness and crate, and how often to check.
Why not buy the adult-size harness now?
- A harness bought to "grow into" is loose now. A loose harness slips and rubs, and it is easier for a puppy to back out of1. That is the opposite of what you want near a road.
- Straps that hang loose on a small body can sit in the wrong place: across the throat at the front, or pressed up into the armpits behind the front legs.
- Even on its tightest setting, an adult size may not close down far enough for a puppy. Every size only adjusts across its own chest range, which you can check in the chart below.
- Growth is hard to predict, especially in crossbreeds. You might buy the "adult" size and find your dog ends up a size either side of it anyway.
How a Border Collie puppy grows
Border Collies are medium-sized dogs. Medium dogs reach adult weight sooner than large breeds but later than toy breeds3. Adults weigh about 12–20 kg2.
We haven't found a UK source we trust for Border Collie puppy weights month by month, so we don't show one. Your puppy's own chest measurement is the number that matters for a harness.
Border Collie puppy tips
- Collie puppies often go through a leggy stage, so re-measure the chest rather than judging by eye.
- An active puppy moves around a lot in its harness. Check for rubbing behind the front legs after walks.
Stage by stage: what to do from 8 weeks to 12 months
These are our suggested checkpoints. They're a routine for measuring, not a growth chart: your puppy's own tape-measure reading always wins.
8 weeks
Measure the chest on the day you bring your puppy home, and buy a harness that fits that number with room to let out. We suggest choosing a style with plenty of adjustment. Let your puppy wear it for short spells at home first, with lots of praise.
3 months
We suggest measuring every two weeks from now. Loosen the straps as needed and check the fit against your brand's guidance each time (Bailey & Coco, for example, says two fingers should fit under any strap4).
4 months
Keep measuring every two weeks. If the straps are already on their loosest setting and the two-finger check is tight, it is time for the next size.
6 months
Compare the chest with the size chart again. Many puppies are between sizes around now. For a growing puppy that is truly between two sizes, Ruffwear suggests the larger one1.
9 months
If the chest has barely changed across two or three measurements, we suggest dropping to a monthly check. If it is still rising, keep to every two weeks.
12 months
Measure again. Small dogs are usually close to adult weight by now, but large breeds can keep growing into their second year3, and the chest often fills out after height stops. Keep checking monthly until the number stops changing.
Signs it's time to change size
- The straps are on their loosest setting and the fit is still snug by your brand's check (Bailey & Coco's is two fingers under any strap4).
- The chest measurement is at, or above, the top of the size's chest range on the chart.
- Red marks, rubbed or broken hairs behind the front legs, under the chest or around the neck. If the skin is sore, stop using the harness and ask your vet.
- The strap behind the front legs has crept up into the armpits, or the front strap now sits on the throat.
- Your puppy walks differently in the harness: a shorter stride, or trying to scratch or bite at it.
When you move up, measure again rather than just picking the next letter. Sizes don't mean the same thing in every brand.
Puppy harness sizes by brand
Each brand's full chest range, smallest first, so you can see where a puppy fits today and where it will go next. Check where each chart starts if you have a small puppy.
Julius-K9
IDC Powerharness
Smallest size: Baby 1, for a chest of 29–36 cm.
Baby 1 29–36 cm · Baby 2 33–45 cm · Mini-Mini 40–53 cm · Mini 49–67 cm · 0 58–76 cm · 1 63–85 cm · 2 71–96 cm · 3 82–115 cm · 4 96–138 cm
Adult Border Collies usually need: Mini or 0.
Check price on AmazonRuffwear
Front Range Harness
Smallest size: XXS, for a chest of 33–43 cm.
XXS 33–43 cm · XS 43–56 cm · S 56–69 cm · M 69–81 cm · L/XL 81–107 cm
Adult Border Collies usually need: XS to M.
Check price on AmazonBailey & Coco
Trail & Glow Harness
Smallest size: XXS, for a chest of 22–41 cm.
XXS 22–41 cm · XS 26–50 cm · S 35–66 cm · M 40–71 cm · L 55–83 cm · XL 71–99 cm
Adult Border Collies usually need: M or L.
Check price on AmazonFor the full charts and adult fit, see our Border Collie harness size guide.
If your puppy is between two sizes
- Pick the size where the chest measurement sits nearer the middle of the range, so you have adjustment either way.
- For a growing puppy that is truly between two sizes, Ruffwear suggests the larger size1. Only do this if the larger size can be tightened to a safe fit now.
- For Julius-K9, check the weight range as well: if your puppy's weight only fits one of the two sizes, go with that one5.
- If the larger size would only just fit on its very tightest setting, choose the smaller one and plan to move up soon. A loose harness is easier to escape.
Fit checks every time you adjust
Snug by your brand's check
Bailey & Coco says you should be able to fit two fingers under any strap4. Julius-K9 instead describes how far behind the front legs the belly strap sits6. Follow your harness's instructions.
Clear of the throat
The front straps should sit on the breastbone, not across the windpipe.
Clear of the armpits
The body strap should sit a little way behind the front legs, not pressed up into them.
Escape test
With the lead clipped on, pull gently forwards, then backwards and upwards. The harness shouldn't slide over the shoulders. Puppies are wriggly, so do this every time you adjust it.
Common mistakes
- Buying the adult size "to save money" and walking a puppy in a loose harness.
- Guessing the size from age or weight instead of measuring the chest.
- Setting the harness once and forgetting it. A puppy can outgrow a fit in a couple of weeks.
- Measuring over a fluffy puppy coat without pressing the tape to the body.
- Leaving a harness on all day and night. We suggest taking it off when your puppy is at home and unsupervised, so it can't snag on anything.
- Forgetting the collar and ID tag. A harness doesn't replace them in public7.
Collar and ID tag
In the UK, a dog in a public place must wear a collar with the owner's name and address on it, or on a tag or plate attached to it7. That applies to puppies too. Ask your vet when your puppy can safely start going out.
Border Collie puppy harness questions
What size harness does a Border Collie puppy need?
Whatever size fits its chest today. Measure around the widest part of the ribcage, just behind the front legs1, and pick the size whose chest range includes that number, ideally near the middle.
Should I buy my Border Collie puppy an adult-size harness to grow into?
No. A harness that is too big slips, rubs and is easier to back out of1. Buy for the chest your puppy has now, and move up when it outgrows it.
How often should I measure my Border Collie puppy?
We suggest every two weeks while your puppy is growing fast, then monthly until its chest measurement stops changing. Check the fit against your brand's guidance every time you put the harness on.
How many harnesses will my Border Collie puppy need?
It depends on how fast your puppy grows and how much each size adjusts. Many owners get through two or three. Adjustable styles with a wide range in each size can reduce the number.
When is a Border Collie fully grown?
Smaller dogs reach adult weight sooner than large ones, and large breeds can keep growing into their second year3. Keep measuring until the chest stops changing, and ask your vet if you have questions about your puppy's growth.
Is a harness or a collar better for a puppy?
My puppy's measurement is below the smallest size. What now?
Some brands start at a larger chest size than others. Check where each chart starts, below, and choose a brand whose smallest size covers your puppy now. Don't use a size that is too big.